In recent years, a so-called droplet-discharging technology is used for print apparatuses, such as printers, and deposition apparatuses for manufacturing semiconductor devices. The droplet-discharging technology allows discharge and throw of droplets, such as ink and deposition material for deposition apparatuses, onto targets.
Enhanced quality of printed images or accuracy of deposition is expected for such a droplet-discharging technology, and accordingly, droplet-discharging heads used for the droplet-discharging technology should have discharge characteristics that enable accurate ejection of very small droplets onto targets. Further, in accordance with recent reduction in size and increase in resolution of droplet-discharging heads, there has been a need for droplet-discharging heads having high-density nozzles to discharge droplets and at the same time having good discharge characteristics. The manufacture of droplet-discharging head substrates with high-density nozzle holes requires high processing accuracy for alignment and bonding of the plates constituting the substrates.
An example of such droplet-discharging head substrates is shown in FIG. 1, which is an exploded perspective view of a droplet-discharging head. The droplet-discharging head substrate 2 is constituted of a nozzle plate 21, an intermediate plate 22, and a body plate 23. The nozzle plate 21 has high-density nozzle holes 211. The intermediate plate 22 has communication holes 221 to communicate with the respective nozzle holes 211 to form flow paths. The body plate 23 has flow paths to communicate with the respective through-holes individually and has pressure chambers communicating with the flow paths and having piezoelectric elements 234 to discharge droplets at relevant positions. Accurate bonding of these plates is necessary for the discharge head to have good discharge characteristics.
A conventional method for bonding the plates uses an adhesive agent to bond the joint surfaces to each other. Bonding with an adhesive agent, however, has a risk that the adhesive agent may cover the openings formed in the plates and thus may affect the discharge characteristics. Such a risk is especially high for a droplet-discharging head substrate having high-density openings, such as nozzle holes and flow paths.
In view of this, there have been bonding methods without using an adhesive agent, such as anodic bonding and surface activated bonding where the surfaces of members are activated at a lower temperature than in anodic bonding for bonding the surfaces to each other.
The anodic bonding is a method using covalent bonding caused by the movement of cation contained in a glass plate. Such a method can bond members tightly without the need for an adhesive agent as disclosed in, for example, Patent Literature 1. In the case of formation of a droplet-discharging head by bonding an Si plate and glass member by anodic bonding as in Patent Literature 1, a high temperature of 300° C. or higher needs to be applied for cation movement while joint surfaces are softened and brought closed to each other.
The surface activated bonding is a method where the joint surfaces of a silicon substrate and a glass substrate are irradiated with an atom beam, an ion beam, or a plasma as an energy wave to be activated for OH or ON groups to be added to the joint surfaces and then the substrates are bonded to each other by atomic bonding between the substrates. Similarly to the anodic bonding, the surface activated bonding can bond joint surfaces with each other without using an adhesive agent as disclosed in Patent Literature 2. Instead of the ion movement requiring a high temperature as in the anodic bonding, the surface activated bonding needs to press the plates against each other with a high pressure while softening the joint surfaces under the above-mentioned low temperature for the joint surfaces to come close to and bond to each other.